22 ON THE FRESH-WATER GLACIAL DRIFT 



liquid part would find its way to the lowest ground, forming local basins of water; 

 and as these increased, the buoyant force would be competent to raise and float 

 the solid portions. 



As the watery portion increased and began to predominate, there must have 

 been general currents, as there are in all large bodies of water. Thus the sorting 

 and stratification of the finer materials can be accounted for. For a time the 

 glacial waters would be muddy, and thus the lighter sediment corresponding to 

 the prairie loam could be transported and deposited over large tracts. 



It is to this period of moderate southerly currents that I ascribe the loess-like 

 deposits of Illinois, Southern Iowa, and Missouri. The preglacial valleys formed 

 a convenient receptacle for these materials in their southerly progress. 



As the recent rivers began to assume the channels of a prior geological era, they 

 cut dovfn rapidly into the soft materials, which were thus again transported to 

 lower levels, forming alluvial deltas of which the one at the mouth of the Missis- 

 sippi is the most conspicuous. This is the epoch of river terraces. 



Glacial StricB. 



The course of the arrows upon the map is fixed by the following table, showing 

 the bearing of grooves and striae at numerous points. Where there is more than 

 one observation, groups have been formed by taking the average of several obser- 

 vations whose bearing lay within the same quadrant. The number in each group 

 embraces a certain contiguous territory which is determined arbitrarily. 



To mark on the map each observation by an arrow is not practicable on this 

 scale, and would convey a less clear idea than the results represented by groups. 

 Those in States eastward of Ohio are taken from published geological reports. 



In the States of Ohio, Michigan, Wisconsin and Minnesota, they are principally 

 from my field-books, making use, in addition, of such as are reported by Prof. 

 Hector of the Pacific Railroad Surveys of Canada, and by Messrs. Foster, Whitney, 

 and Desor. 



